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Stocks et al.
designed a study to assess the ability of different doses of intrathecal fentanyl to reduce requirements for intrathecal bupivacaine administered to women during the first stages of labor. The 124 women recruited for the study were all in labor and dilated between 2–6 cm. They were volume-loaded with saline, and then an intrathecal injection was administered as part of a combined spinal–epidural technique. (The subsequently inserted epidural catheter was not used for this study.) In the first phase of the study, women were randomized (30 per group) to administration of either bupivacaine alone or bupivacaine in combination with 25 μg fentanyl. In the second phase of the study, bupivacaine, in combination with either 5 or 15 μg fentanyl, was administered to the women.

Bupivacaine was administered in either higher or lower doses according to an up–down sequential allocation, with testing intervals set at 0.25 mg bupivacaine. Efficacy of the study drug was assessed using a 100-mm visual analog scale, and effective dose
was defined as the amount of drug resulting in a visual analog pain score of 10 mm or less within 15 min after injection. An effective dose directed a decrement of 0.25 bupivacaine for the next patient randomized to that particular group. An ineffective dose
(visual analog pain score of more than 10 mm, 15 min after administration) directed an increase of 0.25 bupivacaine in the next person in the group. Rescue boluses of 2 μg/ml fentanyl were offered to women who experienced ineffective analgesia.

The researchers recorded statistics, such as weight, height, and parity, for each patient, as well as use of oxytocin and prostaglandin for induction of labor. Sensory levels were assessed by reaction to ethyl chloride spray and to pin prick. The minimum local analgesic dose of intrathecal bupivacaine was calculated at 1.99 mg using the Dixon and Massey formula. There were significant reductions in minimum local analgesic dose for all bupivacaine–fentanyl groups compared with the bupivacaine control group. The addition of 5 μg intrathecal fentanyl showed the same bupivacaine dose-sparing effect as 15 and 25 μg. Increasing fentanyl doses resulted in increased pruritus and duration of spinal analgesia. These findings suggest that analgesia in the first stage of labor can be achieved with lower doses of fentanyl than those commonly used, with a resulting shorter duration of action.